The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Pulsed electrochemical techniques are well known. Pulsed voltammetric techniques may lower detection limits in electrochemical analysis. In a typical electrochemical cell, one observes two types of current flow, which are termed “Faradaic” and “non-Faradaic” currents. Faradaic currents result from the electrochemical conversion of one chemical substance to another, either oxidation or reduction, and are generalized by:Ox+ne−→Red  (1)
The symbol, “e−” in equation (1) represents the electrons transferred in the electrochemical conversion and n is the number of electrons. The transferred electrons in the electrochemical conversion give rise to the output/signal current produced by the sensor when it is exposed to the target gas. On the other hand, non-Faradaic currents result from rearrangement of ions present in the electrolyte of the sensor very close to the electrode surface and other processes such as adsorption and desorption of ions. Non-Faradaic currents do not contribute to the analytical signal of the sensor, but result in the noise observed in the sensor signal when no target gas is present. The above discussion applies to a sensor operated at a constant potential.
In the absence of target gas, if the potential applied to the working electrode of a sensor is suddenly changed, an instantaneous current will flow in the sensor that is non-Faradaic. This current has an exponential time dependence and will decrease toward zero current according to:
                              i          c                =                              E                          R              s                                ⁢                      e                                                            -                  t                                /                                  R                  s                                            ⁢                              C                WE                                                                        (        2        )            where iC is the observed current (the charging current), t is the time after the potential change, E is the magnitude of the potential change, RS is the solution resistance, and CWE is the capacitance of the working electrode (which is directly dependent upon the electrochemically active/accessible area of the working electrode). This behavior is shown in FIG. 1A.
If the potential change is applied to the working electrode when target gas is present, and the potential of the working electrode is such that the target gas undergoes a Faradaic reaction (is oxidized or reduced), the observed current is provided by:iT=ic+iF  (3)
Where iT is the sum of the charging current, iC, and the Faradaic current. iF. In the case of amperometric electrochemical sensors, iF is often expressed as:
                              i          F                =                  nFAC          ⁢                      D            x                                              (        4        )            In equation (4), n is the number of electrons involved in the electrochemical reaction, F is Faraday's constant, A is the electrochemically active/accessible area of the working electrode C is the concentration of the target gas, D is the diffusion coefficient of the target gas, and x is the distance the target gas must diffuse to reach the electrochemically active surface of the working electrode. Equation (4) is obtained by considering Fick's laws of diffusion. Equation (4) indicates that the Faradaic current is directly dependent upon the concentration of the target gas and is assumed insensitive to time, which is an approximation. The value of D/x (the solution to Fick's laws under the physical conditions of the experiment) will always be time dependent. However its value quickly reaches a steady state condition for amperometric electrochemical gas sensors, and is essentially independent of time.
Various discussions and derivations of the theory of pulsed voltammetry or polarography indicate that the motivations behind the development of these techniques include increasing the analytical sensitivity of these methods by separating, in time, the charging current and the Faradaic current. There are at least three critical criteria generally accepted to be required for the success of pulsed voltammetric methods: First, the potential pulse should be small. For maximum fidelity, the pulse magnitude should be less than about 0.059/n volts (at 25 C), where n is the number of electrons transferred in the electrochemical reaction (see equation (1)). Second, the time between pulses should be long, allowing the charging current to decay to small values. For classically sized analytical electrodes (1 cm2) or smaller, the wait time can be on the order of several seconds (that is, longer than 5 seconds and more typically several hundred seconds). Third, the electrode area should be minimized. Pulsed voltammetric techniques were first developed for use in electroanalytical procedures with macro electrodes for which the geometric area closely approximated the electrochemically active/accessible area. See, for example, A. J. Bard and L. R. Faulkner, Electrochemical Methods (Wiley: New York), 1980, 183; P. T. Kissinger and W. R. Heineman, Laboratory Techniques in Electroanalytical Chemistry (Marcel Dekker: New York), 1984, 143; J. Osteryoung and M. M. Murphy, “Normal and Revers Pulse Voltammetry at Small Electrodes,” Microelectrodes: Theory and Applications, 1991, 123-138; and J. Osteryoung and K. Hasebe, “Pulse Polarography—Theory and Application,” Review of Polarography, 1976, 1:22, 1-25. The desire to increase the sensitivity of these techniques (as well as other motivations) led to the development of micro- and ultramicro-electrodes; electrodes with areas of approximately 1 μm2.
All of these developments generally had one goal in mind; minimizing the charging current with respect to the Faradaic current. Pulsed voltammetric techniques lowered the typical useful concentration range of electroanalytical methods from parts-per-thousand (10−3) to parts per million (ppm, 10−6), and lower.
The above discussion applies to classic solution-oriented electroanalytical techniques. Amperometric electrochemical gas sensors differ in several important ways which have been understood to severely limit or eliminate the usefulness of pulse techniques in such an application.